pyldb: Add ldb.disconnect() method to ensure DB handles are closed

pyldb: Add ldb.disconnect() method to ensure DB handles are closed

This is vital in our backup code, which needs to actually close the
LMDB at the correct point.

The Python ldb object itself is left in more or less the same state as
one that has not connected to a server or database (it is a very
simple wrapper in itself), and can be reconnected using the .connect()
method.

Pair-programmed-with: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>